1. Field of the Invention
The present invention relates to an electrical transformer or reactor having at least one winding of tape-formed conductor material and means for reduction of additional losses in the winding by controlling the magnetic leakage flux appearing axially outside the winding ends.
2. Prior Art
In transformers and reactors having windings of foil-formed electrical conductor material, additional losses in the form of eddy current losses appear at the ends of the windings. The substantially axial magnetic leakage flux passing between the winding turns is deflected in a more or less radial direction at the ends of the windings and passes into the core legs or completes its path outside the windings instead of continuing axially and passing into the yokes. This means that the ends of the windings will be penetrated by a magnetic flux with a radial component, which generates eddy currents in the winding conductor and causes additional losses in excess of the unavoidable ohmic losses which are caused by the normal load current. These eddy current losses raise the temperature in the winding ends so that it may locally assume high values.
Several different measures have been proposed to reduce the radial leakage flux component and thus the losses and the locally high temperatures at the winding ends. U.S. Pat. No. 4,021,764 and 4,012,706, for example, propose the arrangement of ceramically bonded ferrite plates and open rings of electrical sheets, respectively, in the vicinity of the edges of the foil winding, which rings are connected to the different turns in the foil winding. From the point of view of manufacturing technique, however, it is desirable, for example as proposed in British patent specification 2,025,148, to construct such magnetic-flux-controlling bodies in the form of coils coaxial with the winding, which coils are wound together with the foil winding. Electrical sheet has been proposed as material for such coils. To limit the voltage between the foil winding and the magnetic coils upon a short-circuit or a voltage surge, the coils must be galvanically connected to the foil winding, at least at the end points of the coils, possibly also at a number of intermediate points. In view of the losses the magnetic sheet from which the coils are wound must have a resistivity in the longitudinal direction of the sheet of at least 10.sup.-4 .OMEGA. m, preferably greater than 5.10.sup.-4 .OMEGA.m. At the same time, the permeability of the sheet perpendicular to the longitudinal direction and in the plane of the sheet must be at least 10, preferably greater than 80, and the saturation value of the sheet should be at least 0.4 T, preferably greater than 0.8 T. None of the materials previously proposed for this purpose fulfills these requirements. Thus, electrical sheet has a resistivity which amounts to only about one-thousandth of the required resistivity, and ferrites have a too low saturation value.